Caching Proxy: Revalidation
GET /foo/index.html
HTTP/1.1
Host: www.example.com
HTTP/1.1 304 Not Modified
GET /foo/index.html
HTTP/1.1
Host: www.example.com
If-Modified-Since:
Thu, ...
Proxy
HTTP/1.1 200 OK
Last-Modified: Thu, ...
Content-Length: 3688
Content-Type: text/html